Apparatuses for polishing the surfaces of semiconductor wafers such as silicon wafers include single side polishing apparatuses polishing one sides of wafers at a time, and double side polishing apparatuses polishing both sides of wafers at a time.
As shown in FIG. 12, a typical single side polishing apparatus is composed of a turntable 73 to which a polishing pad 74 is attached, a mechanism 76 of providing a polishing agent, a polishing head 72, and the like. A wafer W is polished with such a polishing apparatus 71 by rubbing the surface of the wafer W against the polishing pad 74 while the wafer W is held by the polishing head 72, the mechanism 76 of providing a polishing agent 75 to the polishing pad 74, and the turn table 73 and the polishing head 72 are rotated respectively.
Wafers can be held by a method of sticking the wafers on a flat disklike plate with an adhesive such as wax or the like, a method of sticking with water the wafers on a soft pad (a backing pad), a method of sucking the wafers with vacuum, or the like.
FIG. 14 illustrates a schematic example of a polishing head holding a wafer with a backing pad. This polishing head 91 has an elastic pad (a backing pad) 95 made of polyurethane or the like on the underside of a disklike carrier 92 made of ceramic or the like. The pad 95 is dampened with water to hold a wafer W with surface tension. In addition, in order to prevent the wafer W from coming off from the carrier 92, a ring (retainer ring) 94 is provided around the carrier 92.
For example, when a silicon wafer is polished, the retainer ring is typically made of plastic such as phenylene sulfide, which is softer than silicon single crystals for the purpose of preventing generation of scratches or impressions on the edge portion of the wafer and for the purpose of preventing metallic contamination. However, because a polishing agent enters between the retainer ring and the edge portion of the wafer (a chamfered portion) during polishing and the wafer rotates freely during polishing, the wafer comes into contact with the retainer ring, thereby polishing the inner periphery of the retainer ring and forming a groove there. When the edge portion of the wafer is caught in the groove, the wafer being pressed uniformly over the surface by the carrier is further pressed by the retainer ring, and which affects polishing of the outer periphery of the wafer. As a result, there is a problem that the outer periphery of the polished wafer has a degraded flatness level.
Therefore, in order to prevent the degradation of flatness level in the outer periphery of a wafer, it is necessary to change the retainer ring frequently, resulting in increase in cost.
In order to prevent excessive polishing in the outer periphery of a wafer, a polishing head with a tube or a diaphragm for adjusting pressing force of a retainer ring separated from a pressing force adjusting mechanism of a carrier (see Japanese Patent Publication No. 3158934). It is stated that the excessive polishing in the outer periphery of a wafer can be prevented as a result of adjusting pressing force of the retainer ring against the polishing pad by such a mechanism of adjusting pressing force of the retainer ring to prevent waviness deformation of the polishing pad.
In order to prevent a wafer from being unfastened from a carrier during polishing, a polishing head is suggested that has a ring guide around the carrier; a pressure ring for pressing a polishing pad, the pressure ring being outside of the ring guide; and an air cylinder or a roller for adjusting pressing force of the pressure ring (see Japanese Patent Publication No. 3045966).
Typically, the pressure ring is made of a hard material such as ceramic that is less prone to wear and has less frictional resistance against a polishing pad. When the pressing force adjusting mechanism designed specifically for the pressure ring (the retainer ring) is provided, pressing force of the pressure ring can be prevented from directly influencing on pressing force of the wafer. However, both the wafer and the pressure ring are always subjected to strong force in the thrust direction during polishing due to friction against a polishing pad. Typically, a wafer has a coefficient of friction larger than the pressure ring during polishing. When foreign matters such as a polishing agent enters where the edge portion of the wafer comes into contact with the inner periphery of a guide ring that holds the wafer, because the pressing force adjusting mechanism for the wafer and the pressing force adjusting mechanism for the pressure ring are separated and independent of each other, the wafer comes in contact with the pressure ring due to sideways deviation of the wafer, the guide ring or the carrier. As a result, impressions or scratches tend to be generated on the edge portion of the wafer even when the inner periphery of the guide ring holding the wafer is made of plastic.